Alloyed-zinc-plated steel sheets exel ordinary zinc-plated steel sheets in continuous operability in spot welding, in adhesion of electrodeposited coating films and in corrosion resistance. Therefore, they are extensively used in the automobile industry and a wide range of other industries.
Although alloyed zinc-plated-steel sheets are excellent as mentioned above in adhesion of coating films and weldability in spot welding, they are generally inferior in workability because of the brittleness of intermetallic compounds formed in the alloyed layer. In particular, alloyed-zinc-plated steel sheets manufactured by the conventional hot-dip plating and electroplating are found to suffer marked powdering as the coating weight and the Fe content in the alloy layer increase.
Meanwhile, continuous vacuum vapor deposition zinc plating has now come into practical use in addition to the conventional hot-dip plating and electroplating, and manufacture of alloyed metal-plated steel sheets by the vacuum vapor deposition process is also being attempted. The continuous vacuum vapor deposition plating process is especially superior to the conventional hot-dip plating and electroplating in that it enables easy high speed manufacture of plated sheets, both single-side plated and double-sized plated sheets with plating thickness ranging from thin to thick or different thicknesses double-side plated sheets.
We have found that if vacuum vapor deposition Zn plating is carried out in an atmosphere containing oxygen controlled to a prescribed level, the alloyed layer is minutely cellulated whereby powdering of the resulting alloyed layer is largely prevented.